This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2001-88694 filed on Mar. 26, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a solenoid valve in which pressure of a control pressure chamber provided between a spool and a solenoid is controlled to change over a communication between a first supply port and an output port to a communication between the output port and a first drain port by oil supplied from a second supply port to the control pressure chamber and ejected therefrom to a second drain port in cooperation with the solenoid.
2. Description of Related Art
Recently, a hydraulic circuit of an automatic transmission for a vehicle has a two ports-current proportion solenoid valve (so called a bleed type-current proportion solenoid valve) in which oil pressure of a control pressure chamber between a control orifice and a seat member is adjusted by controlling a flow amount of oil to be ejected from the seat member.
The conventional solenoid valve has a stator core and a moving core slidably movable along the stator core upon energizing a coil. The moving core is provided at an axial end thereof with a bleed valve body that is normally in contact with a bleed valve seat of the seat member. When the coil is energized, the moving core moves so that the bleed valve body is away from the bleed valve seat and a bleed hole is opened. Accordingly, the oil of the control pressure chamber is ejected through the bleed hole to a drain so that the oil pressure of the control pressure chamber is reduced.
However, according to the conventional solenoid valve, when the bleed hole of the seat member is opened to reduce the pressure in the control pressure chamber, a flow amount of oil to be ejected from a drain port is large and the large flow amount thereof continues until the bleed hole is closed again, which gives a heavy burden on a hydraulic pump for supplying the oil to the control pressure chamber so that fuel consumption is worse.
To cope with this disadvantage, there has been proposed a solenoid valve in which a diameter of the control orifice is smaller so that a less flow amount of oil is ejected or leaked from the drain port when the bleed hole of the seat member is fully opened.
However, the smaller diameter of the control orifice has a disadvantage that, at low temperature, the oil can not flow smoothly and a hydraulic response characteristic is deteriorated thereby.
An object of the invention is to provide a solenoid valve in which pressure of a control pressure chamber can be swiftly reduced and, after reducing the pressure to a target pressure, a flow amount of oil to be ejected or leaked from a drain port is small.
It is an aspect of the present invention that the solenoid valve is manufactured at less cost.
To achieve the above object, in a solenoid valve having a hollow sleeve and a spool slidably moving in the hollow sleeve, a seat member fixed to the hollow sleeve has an oil hole whose one end communicates with a control pressure chamber formed in the hollow sleeve between the seat member and the spool. A supply passage through which a second supply port communicates via a first control orifice with the control pressure chamber and a drain passage through which the other end of the oil hole is able to communicate with the second drain port are provided therein. A solenoid has an armature core movable according to an electromagnetic attracting force. The spool is moved in a direction away from the seat member by pressure increase of the control pressure chamber due to oil supplied thereto via the supply passage, when the armature core is in contact with the seat member and closes the other end of the oil hole, and moved toward the seat member by pressure decrease of the control pressure chamber due to the oil ejected therefrom via the drain passage, when the armature core is away from the seat member and opens the other end of the oil hole, so that a communication between a first supply port and an output port is changed over to a communication between the output port and a first drain port and vice versa.
In the solenoid valve mentioned above, a second control orifice, whose cross sectional opening area is smaller than that of the first control orifice, is formed in the supply passage when an end of the spool comes in contact with an end of the seat member so that a flow amount of the oil supplied to and ejected from the control pressure chamber through the supply and drain passages after the spool contacts the seat member is smaller than that before the spool contacts the seat member.
According to the solenoid valve mentioned above, once the spool comes in contact with the seat member, the flow amount of oil supplied to and ejected from the control pressure chamber is small due to the second control orifice, which serves to improve a fuel consumption of a vehicle, for example, if applied to an automatic transmission.
It is preferable that the second control orifice is a slit or an aperture formed at a periphery of at least one of the end of the seat member and the end of the spool that come in contact with each other so as to sandwich the control pressure chamber therebetween so that one end of the slit communicates with the control pressure chamber and the other end thereof communicates with the first control orifice. This construction is more beneficial when the first control orifice is located in a vicinity of a portion where the seat member contacts the spool since the second control orifice can communicate with the first control orifice with a shorter distance.
Further, since the slit or aperture is simple in construction, the solenoid valve can be manufactured at less cost.
If the first control orifice is located away from the portion where the seat member contacts the spool, the supply passage may have an oil conduit formed in the spool, whose one end communicates with the first control orifice and whose the other end communicates with the control pressure chamber and, further, the seat member may be composed of a first seat member, positioned on a side of the solenoid, having the oil hole and a second seat member, positioned on a side of the spool, protruding into the control pressure chamber and having a slit constituting the second control orifice through which the other end of the oil conduit is opened to the control pressure chamber, when the spool comes in contact with the second seat member so as to sandwich the control pressure chamber between the spool and the first seat member.
Further, in a case that the supply passage is provided with an oil conduit formed in the spool, whose one end communicates with the first control orifice and whose the other end communicates with the control pressure chamber, it is preferable that the one end of the oil conduit opened to the first control orifice constitutes the second control orifice when the spool comes in contact with the second seat member so as to sandwich the control pressure chamber therebetween and closes partly the first control orifice.